Many types of electrode can be used in an alkaline electrolyte storage cell, such as sintered electrodes and non-sintered electrodes, also referred to as paste electrodes or plasticized electrodes. Compared to other types of electrode, a non-sintered electrode contains a greater quantity of material and its capacity per unit volume is therefore increased while its manufacturing cost is reduced.
A non-sintered nickel electrode includes a support serving as a current collector coated with a paste containing both the active material and a binder, to which paste a conductive material is usually added. Said support is a porous three-dimensional conductive support such as felt or foam based on metal or carbon; it can also be a two-dimensional conductive support such as expanded metal, mesh, woven fabric, or solid or perforated strip. While the electrode is being manufactured, a volatile solvent is added to the paste to adjust its viscosity to facilitate shaping it. When the paste has been deposited on or in the support, the result is compressed and dried to obtain an electrode of the required density and thickness.
In a non-sintered nickel electrode, the active material is a nickel-based hydroxide. Nickel hydroxide is a poor conductor and requires a material enabling good electrical percolation to be added to the electrode. The paste therefore generally contains a conductive material, for example a cobalt compound such as metallic cobalt Co, cobalt hydroxide Co(OH).sub.2, and/or cobalt oxide CoO. The first time the alkaline storage cell is charged, these compounds are oxidized to form cobalt oxyhydroxide CoOOH in which the degree of oxidation of the cobalt is greater than or equal to +3. Cobalt oxyhydroxide is stable in the normal range of operation of the nickel positive electrode and is insoluble in the alkaline electrolyte. It provides the electrical percolation of the electrode.
If an alkaline storage cell having a non-sintered nickel-positive electrode is stored in a completely discharged state, its voltage decreases with time. If the battery is stored for more than a few months, its voltage tends to 0 V. Under those conditions, the cobalt oxyhydroxide is slowly reduced. The degree of oxidation of the cobalt initially falls to +2.66 (Co.sub.3 O.sub.4) and then to +2 (Co(OH).sub.2).
Cobalt hydroxide Co(OH).sub.2 is highly soluble in the electrolyte. After several months in storage there is therefore a loss of conductivity due to partial dissolution of the percolating structure of the non-sintered electrode. This causes an irreversible loss of capacity, which can exceed 15%, and which occurs regardless of the cobalt compound in the paste.
European Patent Application EP-A-0 798 801 and U.S. Pat. No. 5,672,447 propose the use of nickel hydroxide powder coated with a disordered cobalt compound having a valency greater than +2. European Patent Application EP-A-0 789 408 specifies a cobalt compound of that kind containing from 0.1% to 10% by weight sodium. Such coatings are likewise not stable during storage at a low voltage.